Two charities have expressed concern about plans, unveiled in the Budget, to extend a new eligibility test to all incapacity benefit recipients.
There are 2.64 million people claiming incapacity benefit in the UK
The Disability Alliance said training in new skills should be offered rather than "throwing" people off benefits.
Meanwhile Disability Action claimed any government measures should not just be "a matter of putting a figure on it".
Minister James Purnell said the idea was part of "a radical reform package" to further extend "incentives to work".
The work and pensions secretary added this could also help to move more children above the breadline.
"We will require lone parents with older children to look for work, and that will lift another 70,000 children out of poverty," he said.
The government has set a target of having 70% of single parents in employment by 2010.
Mr Purnell also pledged "a radical reform package to further extend and improve opportunities and incentives to work, lift even more children out of poverty and give independence, choice and control for disabled people".
But the Tories, who have already promised a similar scheme, said not enough money was being allocated for the tests.
Chris Grayling, the shadow work and pensions secretary, branded the government's proposals "a complete fraud".
"Where is the programme? Where is the support? Where is the plan to get people back into mainstream employment?".
"There is none of that," he told the BBC News website, adding it was "a kneejerk announcement about tests without any timeline and without any money".
There are 2.64 million people on incapacity benefit in the UK - the lowest level in more than seven years - but the government wants to cut this figure by about a million by 2015.
From this autumn incapacity benefit will be renamed as the employment and support allowance, and all new claimants must complete an assessment considering their capability to work.
But Chancellor Alistair Darling went further in Wednesday's Budget, insisting that all current incapacity benefit claimants would have to take this test from April 2010.
It has been estimated that over three years, at least 50,000 people would fail the assessment.
Paul Treloar, the director of policy and services at the Disability Alliance, said that his charity supported efforts to help disabled people into work, but said it was "dishonest" to use a more stringent test simply to reduce the number of claimants.
Moving the bar higher to claim incapacity benefit was not a solution, he told the BBC News website.
"When people have been on incapacity benefit for significant periods of time, simply moving them to the jobseekers' allowance is not going to help them back into work."
And he said that, in his experience, "it does cause concern when people think there are intentions simply to throw them off benefits".
The director of services at Disability Action, Kevin Doherty, added that the "overall ethos" of the vast majority of those claiming incapacity benefit was the desire to work.
He called for "creative programmes and initiatives to be introduced to the overall working environment".
It would help if the government was to work with employers to bring people into work for a few hours each day at first, he told BBC News 24.
George Osborne, the shadow chancellor, said that the £10 million set aside in the Budget for the assessment was nowhere near enough to cover its costs.
"They are, at the moment, allocating a very small sum of money to pay for these reassessments, which works out at around - as far as we can tell - £3.80 per person.
"That is barely enough to pay for their tube fare or train fare to the interview," he told BBC Radio 5 Live's Breakfast programme.
But Mr Purnell responded during a Commons debate that the Conservatives "have said very clearly that their goal is the same as ours - that they would take a million people off incapacity benefit".
But they had not come up with any alternative way to fund or undertake this, he said.
Vince Cable, the Liberal Democrats' Treasury spokesman, accused Labour and the Conservatives of "a lot of dishonesty and naivety here" in "imagining people who want to work are being barred from doing it because of the benefits system".
Many of those claiming incapacity benefit were mentally ill, he told BBC News 24, and were unable to seek employment.
And there should be "a more flexible approach to benefits so people can work part-time and are not penalised for it", he added.